How to find the best SIM-only deal

·6-min read

SIM-only mobile deals can be a cheaper and more flexible way to use your phone, provided you don't need or want a new handset. With lots of SIM-only deals on the market, there are several things to bear in mind when tracking down the best one for you.

Here’s how SIM-only deals work, who should have one, and the factors to be aware of before signing up for one.

What is a SIM-only mobile contract?

There are two types of pay-monthly mobile phone contracts: SIM-and-phone and SIM-only. Both give you monthly allowances for calls, text messages and downloads, but while SIM-and-phone deals provide you with a handset, SIM-only deals give you a SIM card to use in a handset that you already own.

With SIM-and-phone contracts, your monthly bill covers the cost of your new handset and your network usage. With SIM-only, however, there's no handset to pay for, which means they're typically cheaper when compared like-for-like.

Related: Compare SIM Only Deals

Is there a catch?

Only that you already need a handset you're happy with in order to take advantage. It could be that you've finished paying off your current handset as part of an old contract, or you’ve simply bought one outright. Going forward, it’s then just a case of paying for network usage.

What are the benefits of a SIM-only deal?

The upshot is that SIM-only deals tend to be more flexible than SIM-and-phone deals. Some of them can be cancelled at just a month's notice as part of a rolling agreement - unlike SIM-and-phone contracts that can tie you in for 12, 24 or even 36 months.

Also, applying for a SIM-only deal involves a less rigorous check of your credit file than a SIM-and-phone deal. Since the mobile network doesn't need to loan you the cash to cover a new handset, it doesn't need to check how you handled credit repayments in the past. This can be a bonus if you have a less-than-perfect credit history.

If that is the case, paying your bill in full and on time each month can actually help you to rebuild your credit rating.

How do SIM-only deals work?

After signing up for a SIM-only deal, you'll receive a SIM card to insert into your phone that you can begin using right away, provided it's compatible with your handset.

First there's physical compatibility to consider, because SIM cards come in three sizes and you'll need one that fits in your phone's SIM tray.

The network that sells you your SIM-only deal will be able to provide you with a standard, micro or nano-SIM card, depending on which one you need. The majority of modern smartphones use micro or nano-SIM cards, but a quick internet search will tell you which your handset requires.

Next there's network compatibility. Some handsets are locked to a particular network and will not accept SIM-cards associated with other networks without first being unlocked.

Unlocking a SIM

Should you find your handset is locked to a particular network, you can get a Network Unlock Code (NUC) from it to make your phone compatible with your new SIM card. In some cases the network operator can unlock your handset remotely. Unlocking is sometimes free of charge, but some networks will charge you up to £20 for the service.

There are also websites and shops that will unlock your handset for a fee, but it's a good idea to check reviews to make sure you're dealing with a reputable firm before handing over your handset or cash.

For the technically-minded, there are also online DIY guides to unlocking your handset. You should only attempt this if you're comfortable with the technical challenge and the risk of damaging your handset beyond repair.

Whichever you choose, the unlocking process generally takes between three days and a month, depending on the network. So, it's best not to sign up for a SIM-only deal until you're sure your handset will accept the new SIM card.

One last thing to be aware of is that unlocking a handset will void any warranty associated with the device, which means you won't be able to return it to the manufacturer for repair if something goes awry.

Can I keep my old phone number?

Yes, your old network can give you a PAC (Porting Authorisation Code) number to give to your new network and they'll transfer over your phone number.

How to find the best SIM-only deal

There are five key things to consider when you compare SIM-only mobile phone deals: Price, contract length, text allowance, minutes allowance and data allowance.

The first two simply cover how much you want to pay each month and for how long you're tied into the contract. Contracts with longer terms tend to benefit from lower monthly bills, but of course they're less flexible. You should consider which is more important to you and choose a deal that strikes the best balance between the two.

How do allowances work?

Allowances are measured per minute for calls, per hundred for text messages and per gigabyte (GB) for data. What you need will depend on how you use your phone.

Minute allowances: Minutes allowances are easier to understand by simply dividing the amount by the days in a month. For example, a SIM-only deal with an allowance of 500 minutes gives you roughly 17 minutes of talk-time a day. Similarly, a text allowance of 500 messages gives you around 17 texts a day. If that seems low, you'll be better off finding a deal with higher allowances.

Many SIM-only deals come with unlimited calls and texts, so you don't need to worry about running out, but they're the contracts that tend to cost more than those that are capped. Remember, modern messaging services like WhatsApp send and receive data via wifi or mobile, which means they don't count towards your monthly text allowance.

Data allowances: Data allowances are a little more difficult to judge. Again, your ideal allowance depends on how you use your phone. The great thing is, many of today's phones will tell you how much data you've used during a given period, so you can always go into your settings and find out how much you typically use in a month to inform your decision.

If you don't have access to those kinds of tools, however, consider how you use your phone. If you're streaming a lot of video via apps like Netflix and YouTube while disconnected from your wifi, you're going to eat through your data allowance quickly. Consider a 2-3GB allowance at minimum.

If, however, you're usually just sending and receiving photos and short clips via WhatsApp, you'll probably be good with a 2GB data allowance or less.

Anything else to look out for?

Some SIM-only deals come with 'free' gifts like shopping vouchers, but that's a false economy if they're not competitively priced - so try not to be won over by such bonuses without first checking the fine print.

Finally, some deals offer cashback that effectively lowers your monthly bill - but that tends to involve some retrospective form filling and providing receipts to evidence your claim. For some, that may not be worth the hassle.

Related: Compare SIM Only Deals